In recent years, as a technology for performing time synchronization between communication devices, phase synchronization by a packet (for example, IEEE (Institute of Electrical and Electronics Engineers) 1588 etc.) has attracted attention instead of using a conventional technology of a GNSS (Global Navigation Satellite System) etc. for performing it. In IEEE 1588, a PTP (Precision Time Protocol) is defined as a protocol for accurately synchronizing times between communication devices in a network.
Generally, between a master device as a communication device that is a synchronization source of a time and a slave device as a communication device that is a synchronization destination of the time, relay devices that relay a PTP packet are installed. Here, in the case of the phase synchronization by the packet, large fluctuations are generated in transmission delay time by processing in the relay devices. Consequently, there is a Transparent Clock function as a function to reduce fluctuations in transmission delay at the time of relay in a communications network of an IEEE 1588 version 2.
In the Transparent Clock function in the master device, a retention time of the relay device is set in a correction field of an overhead of the PTP packet as a correction value of a time. Specifically, the relay device subtracts from the correction value a time (a count value or the like) at the time of the start of its processing, and adds to the correction value a time at the time of the end of its processing. Additionally, in the slave device, an arrival delay time from the master device to the slave device is corrected using the correction value in the PTP packet received from the master device.
Note that Patent Literature 1 discloses a technology in which in a case of relaying communication in multiple stages by a plurality of relay devices (slave devices) between a master device and a slave device that perform time synchronization, time synchronization is performed between the respective relay devices. In the technology according to Patent Literature 1, each slave device writes a measurement time (a processing delay time) of its internal processing in a measurement frame directed from the master device to the terminal slave device, and relays it.